This invention relates to electronic musical instruments generally and more particularly, to an improved vibrato system for electronic organs.
In pipe organ practice, an enhanced animation or a brightening effect for notes played on the organ is produced by playing two or more closely tuned tones together. For example, one set of pipes can be accurately tuned while a second set of pipes is tuned slightly sharp or slightly flat. This enhanced effect may also be produced by sounding together sets of pipes which are purposely tuned slightly flat and slightly sharp so that the true frequency of the note represented by the key played by the organist is between the frequencies of the two sets of pipes. This effect has a slow "beating" associated with it which depends on the frequency spread between the tones being played.
Electrical signals generated to simulate pipe organs can be similarly enhanced by combining both a slightly flat signal and a slightly sharp signal. Various techniques are available to simulate such enhancement without actually generating all the different frequency signals which would otherwise be required. For example, music signals can be passed through a delay line and various output taps along the line can be scanned in both a forward direction and a backward direction through the delay line to create the flat tones and the sharp tones respectively. It is also possible to generate an enhanced output signal using a controlled delay circuit. By modulating a control or clock signal which controls the amount of delay of the circuit, the delay can be slowly increased to generate a slightly flat signal and the delay can be slowly decreased to generate a slightly sharp signal. For this arrangement, two channels are provided with control signals which increase the delay on one channel and decrease the delay on the other channel. The two control signals repeatedly reverse such that one channel is always generating a flat signal while the other channel is always generating a sharp signal. By combining the output signals of the two channels, an enhanced output signal is generated.
One well-known electronic delay circuit commonly referred to as a "bucket brigade" circuit passes sampled input signals along a chain of capacitors via intermediate electronic switch circuits. The amount of delay through the delay circuit is controlled by the frequency of the cotrol or clock signal applied to the electronic switch circuits. If two such delay circuits are driven by two separate clock signals one of which is increasing in frequency while the other is decreasing in frequency, then the output signal of the one delay circuit is slightly sharp while the output signal of the other is slightly flat. The direction of frequency change of the two clock signals is repeatedly reversed such that each signal alternately increases and decreases in frequency and the direction of frequency change is always opposite for the two clock signals, i.e., one clock frequency is increasing while the other clock frequency is decreasing. By combining the output signals from the to channels an enhanced signal is generated.
Vibrato is a pulsating effect produced by periodic alternation of the pitch of a given tone at a rate of approximately six times per second. Illustrations of vibrato are the slight oscillation of a finger on a violin string or the slight wavering of a tone in singing.
A vibrato effect can be imposed upon the output signal from the "bucket brigade" delay circuits by frequency modulating the two control signals controlling the delay of the circuits with a vibrato signal. The increase and decrease in the frequency of the control signals is thus staggered by the vibrato signal such that a warble effect or vibrato is introduced into the output signal.
A control signal for a "bucket brigade" delay circuit can be generated by a voltage controlled oscillator (VCO). If a VCO is driven by a voltage signal which alternates between maximum and minimum amplitudes at a low frequency of approximately one cycle per second or one cycle per several seconds, an output signal which repetitively increases and decreases in frequency is generated. If the voltage signal is mixed with a lower amplitude vibrato signal which varies at a rate of approximately six or seven cycles per second and the mixed signal is used to drive the VCO, the increase and decrease in the frequency of the VCO output signal will not be smooth but will vary at the rate of the vibrato signal. Such a varying frequency signal when used to drive the "bucket brigade" delay circuits as previously described will generate a vibrato effect in a music signal which is passed through the delay circuits.
A difficulty arises in mixing the primary voltage signal to a voltage controlled oscillator with the lower amplitude vibrato signal. When a constant amplitude vibrato signal is mixed with the primary voltage signal, the vibrato effect is accentuated at the low voltage levels of the primary voltage signal and practically nonexistent at the high voltage levels. This is because the small amplitude vibrato signal is large relative to the amplitude of the primary voltage signal at the low levels but at the high levels of the primary voltage signal, the amplitude of the vibrato signal is so small relative to the amplitude of the primary voltage signal as to be insignificant.